This invention relates to a high pressure metal halide discharge lamp of the sodium scandium type and more particularly to a metal halide discharge lamp having improved maintenance during its life.
The high pressure sodium-scandium discharge lamp is well known and widely used because of its relatively high efficiency and good color rendition. The basic concept of this type lamp is disclosed in U.S. Pat. No. 3,407,327, issued Oct. 22, 1968 to Koury et al. and is basically a discharge lamp containing predetermined quantities of mercuric iodide, sodium iodide, mercury and scandium metal in the discharge sustaining fill.
As with most discharge lamps, lamp efficiency tends to diminish somewhat during life. This less than desirable lamp maintenance, in part, results from the fact that a dark film tends to form on the arc tube body. Spectrographic analysis of this black residue discloses a number of metal impurities. Surprisingly, it has been found that the addition of a relatively small quantity of lead metal powder to the discharge sustaining fill will improve the maintenance of the standard sodium-scandium lamp.
Lead in the form of lead iodide has been added to the discharge sustaining fill of high pressure vapor discharge lamps in the past. One such teaching of the addition of lead iodide to the discharge sustaining fill of a high pressure discharge lamp can be found in U.S. Pat. No. 3,513,344, issued May 19, 1970 to D. A. Larson. Prior discharge lamps containing the lead iodide additive were primarily for photocopying processes and a light source for illuminating fluorescent signboards and contained a discharge sustaining fill which was in excess of 25 wt.% lead iodide. These lamps also had rather low efficiencies in a range of from 22 to 32 lumens per watt. A typical 1,000 watt sodium-scandium lamp will have an efficiency of about 100 lumens per watt.